1. Field of the Invention
The present invention relates to printed circuit boards, and particularly to suppression of simultaneous switching noise in a printed circuit board.
2. Description of Related Art
Multi-layer printed circuit boards are broadly used in electronic devices. A conventional four-layer printed circuit board (PCB) as shown in FIG. 1 includes two signal layers 1, 2, a power plane 3, and a ground plane 4. The power plane 3 and the ground plane 4 are disposed between the signal layer 1 and the signal layer 2.
As the density of integrated circuits on PCB's increase, the problems associated with signal switching noise become greater. Many integrated circuits have a large number of input/output (I/O) drivers. Several hundred drivers may be present in some integrated circuits, and this number will undoubtedly become greater as packaging density increases in the future. Noise that occurs in the PCB when a large number of drivers simultaneously switch from one state to another is known as simultaneous switching noise (SSN). The problem may be even more acute when the drivers all switch in the same direction (e.g. high to low). When a large number of drivers switch simultaneously, various signal integrity problems may occur. For example, SSN from a given chip on the signal layer 1 is transmitted to power plane 3 and the ground plane 4 via transmission lines of a power supply and a ground of the given chip. Then the SSN diffuses in the power plane 3 and the ground plane 4. Receivers of other chips on the signal layer 1 may receive the SSN and take incorrect actions.
What is needed, therefore, is a PCB with reduced SSN therein.